Humans and many animals have binocular vision provided by two eyes which look in the same direction. Two parallel aligned but spaced eyes deliver two slightly different images of the same scene. This is due to the 4 to 6 cm separation between the eyes, which makes each eye have a slightly different viewpoint. The images from these two different viewpoints are sent to the brain, and this difference, which is termed parallax, is interpreted as depth. The nearer an object is to the eyes, the greater the difference between the two images. From the difference between the two images, the brain can display an impression of depth.
It is understood that depth perception, or the capacity to perceive the environment in three dimensions, is based on the fact that the right and the left eye of a viewer observe the environment from a slightly different point of view. Therefore, the images perceived by the right and the left eye differ from each other to some extent, and the difference between the images makes it possible to perceive the surrounding space in three dimensions.
Various schemes for displaying moving three dimensional images are known. One common stereoscopic display technique simultaneously displays two images which are encoded for the left eye and right eye by means of different optical polarisations, or colours (e.g. red and green). A viewer wears a pair of special glasses which have filters in front of the left and right eyes. The filters are arranged to influence the way each eye sees the relevant image on the display, i.e. the filters are arranged to pass only the image intended for that eye, i.e. a left eye sees only the image intended for that eye. The filters may be polarisers or include colour filters such as red and green filters.
Another stereoscopic display technique sequentially presents an image intended for the left eye, and an image intended for the right eye. A user wears a special pair of glasses which are shuttered in synchronism with the displayed images, such that the left eye shutter is open during the period when the left eye image is displayed, and the right eye shutter is open during the period when the right eye image is displayed. The frequency of the shutter is adapted to the frequency of the displayed images. The shutters can be mechanical, or can be based on an electro-optical technology such as liquid crystal panels. The quality of the stereoscopic effect is dependent on the extent to which the left eye and right eye images are separated. It is undesirable that any part of a right eye image should be viewed by the left eye, or that any part of a left eye image should be viewed by the right eye.
WO 03/003750 describes a stereoscopic display system using shuttered glasses. Two video projectors are used, and the output of each projector is shuttered in synchronism with the glasses worn by a viewer. When the right-eye shutter of glasses worn by a viewer is open, a shutter associated with the output of the projector providing the left-eye image is shut. Similarly, when the left-eye shutter of glasses worn by a viewer is open, a shutter associated with the output of the projector providing the right-eye image is shut. It is described how this can ensure a good isolation between the left and right eye channels but it has a considerable disadvantage of requiring two projectors, which increases the cost of the apparatus and also requires the pair of projectors to be accurately aligned with one another.
Projectors for video images originally used cathode ray tubes (CRT), but more recently other technologies have been developed where a light beam is applied to an array of selectable light valve elements (also called a spatial modulator). The light valves can use a transmissive technology, such as liquid crystal panels in which elements are individually switched to a level of opaqueness depending on the brightness required for a pixel of the image at that location. Other technologies are based on reflecting light from an array of light valve devices, such as Liquid Crystal On Silicon (LCOS) or Digital Light Processing (DLP).
It is desirable to provide a stereoscopic display using a single projector as this reduces the cost of the apparatus and overcomes the problems of aligning multiple projectors. However, if a single projector having a single light valve array is used, the array is required to quickly change between displaying a left eye image and displaying a right eye image. Some light valve technologies—especially those based on liquid crystal—cannot be instantly switched between displaying a first image and a second image but, instead, have a delay of the order of milliseconds. WO 03/003750 acknowledges this problem in proposing a shuttered two-projector solution.